linq查询将特定数目的列追加到字符串中

Question

I have this linq code:

db.Accounts.Where(o => o.name.Contains(prefixText.Trim())).Select(o => (o.name + ":" + o.account_id)).Take(5)

Now lets say the actual result is 1000 but I only took the top five result.

My question is, will all the 1000 result (name and id) will get appended first and then take the top 5 result?

If yes then how can I modify it to only append the top 5 result not the whole query results?

Answer 1

I have no answer to your question but to make sure you don't concat all 1000 records, you could change you LINQ as

db.Accounts.Where(o => o.name.Contains(prefixText.Trim())).Take(5).
    Select(o => (o.name + ":" + o.account_id)));

Answer 2

When done on a database, that will, of course, be up to the database implemention, although I think it's most likely that the database engine would first choose the records it wants (ie, just the top 5), and then format them based on the select.

When this is performed on an in-memory collection (eg a List<>), then you can be assured that the Select if definitely only be executed 5 times (at most).

It basically works on a "pull" model: Assuming you have something like a .ToList() at the end to actually drive it, The ToList asks the Take() for the first item, and the Take asks the Select, which asks the Where which asks the collection. The where keeps asking the collection until it finds one that meets the condition, and then it passes that the Select which passes it to the Take which passes it to the ToList() which then asks Take for another, and the cycle starts again. After Take has returned the fifth, it tells the ToList , "OK ,We're done. Stop asking for more".